Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogenic and angiogenic inducer that mediates its effect through at least two high affinity-binding receptor tyrosine kinases, Flt-1 and KDR, which are expressed only on the surface of vascular endothelial cells. Flt-1 is required for endothelial cell morphorgenesis whereas KDR is involved primarily with mitogenesis. Gene knockout studies have shown that both Flt-1 and KDR are essential for the normal development of the mammalian vascular system despite their distinct respective roles in endothelial cell proliferation and differentiation. Both Flt-1 and KDR tyrosine kinase receptors have seven immunoglobulin-like (Ig-like) domains which form the extracellular ligand-binding regions of the receptors, a transmembrane domain, and an intracellular catalytic tyrosine kinase domain.
VEGF plays a critical role during normal embryonic angiogenesis and also in the pathological development of new blood vessels in a number of diseases including cancer and in aberrant angiogenesis such as hemangiomas. Solid tumors use blood vessels to obtain oxygen and nutrients and to remove waste materials. In addition these tumors produce stromal factors that induce the formation of new blood vessels to support the tumors' continued growth. Therefore an anti-angiogenic approach represents an attractive and feasible cancer therapeutic option, for example, by inhibiting the VEGF signaling pathway.
One strategy of blocking the VEGF signaling pathway is to sequester away circulating serum VEGF using VEGF binding proteins. VEGF binding proteins serve as decoy receptors working to reduce the amount of circulating VEGF ligand. VEGF binding proteins include e.g., humanized monoclonal antibodies, soluble VEGF receptors, or chimeric VEGF-trap molecules. Chimeric VEGF traps containing anywhere from two to seven of the extracellular Ig-like domains in various combinations have been described (Holash J. et al. Proc. Natl. Acad. Sci. USA 2002, 99:11393-98; Davis-Smyth T. et. al. EMBO 1996, 15:4919-4927; U.S. Pat. No. 6,100,071; U.S. Pat. No. 7,087,411), these VEGF-traps are not optimally effective because they vary in their molecular sizes, VEGF-binding affinity, anti-tumor activity, and pharmacokinetics in vivo. These VEGF-traps are currently administered systemically, and multiple administrations are required in order to maintain a sustained delivery for the VEGF-traps to be therapeutically effective. A gene-therapy approach for sustained delivery of VEGF-traps has been attempted using adenoviruses but the method has been hampered by tissue toxicity issues and low expression of the transgene.